This invention relates to improved Lithium-rich Nickel-Manganese-Cobalt cathode materials for rechargeable lithium ion batteries. The cathode material with layered structure contains Ni, Mn, Co, is doped with metal elements and has a modified composition that shows improved cycle stability, especially an improved high voltage stability, when charged to 4.6 V in a coin-cell type test. High voltage stability is one of the most critical issues of Lithium-rich Nickel-Manganese-Cobalt cathode materials.
Commercially available lithium-ion batteries typically contain a graphite-based anode and cathode materials. A cathode material is usually a powderous material capable to reversibly intercalate and de-intercalate lithium. In the current industrial market, LiCoO2 (so called “LCO”), Li(NixMnyCoz)O2 (so called “NMC”, x+y+z=1), and LiMn2O4 (so called “LMO”) are the dominating cathode materials for rechargeable lithium batteries. LiCoO2 was first introduced as cathode materials for lithium-ion batteries in 1990 by Sony. Since then, it is the most widely used cathode material, especially after commercialization of high voltage LCO, it dominates the application for portable electronics, for example, smartphones and tablets. “NMC” was developed around 2000—to replace LCO—by substituting Co by Ni and Mn, due to high price of Co metal. “NMC” has a gravimetric energy density comparable to LCO, but a much smaller volumetric energy density due to its lower volumetric density. Nowadays, NMC is mainly applied for automotive applications, for example, electrical vehicles (EV) and hybrids (HEV). This is because NMC is much cheaper than LCO, and automotive applications require a smaller volumetric density than portable electronics. “LMO” materials has been developed since the middle of 1990s. LMO has a spinel structure which has “3D” diffusion path of Li-ions. It has been widely used in various applications, such as power tools, E-bikes, and as well as automotive application.
With the rapid development of lithium-ion battery technology and related applications, there is a continuous demand to increase the energy density of the cathode. One approach is to increase the specific capacity of cathode materials. In 2000, a new type Lia(NixMnyCoz)O2 (x+y+z=1, and a>>1) was developed.
Compared to normal NMC, such material has more than one Li atom per molecule. It is usually called “lithium-rich NMC” or “Over-lithiated lithium transition metal oxide” (here we call it “lithium-rich NMC”). Lithium-rich NMC also has a layered structure. The structure is a solid solution of a layered structure with a space group of R3-m and with a certain amount of long-range Li ordering with a √{square root over (3)}ahex×{right arrow over (3)}chex superstructure in the transition-metal layer. Lithium-rich NMC has a very high 1st charge capacity of more than 300 mAh/g with a large plateau in the voltage profile at about 4.5 V (vs. Li/Li+). This plateau is thought to be related to the release of O2. The normal reversible specific capacity is above 250 mAh/g when cycled between 2.0˜4.6 V (vs. Li/Li+), which is much higher than the usual NMC materials. Therefore, lithium-rich NMC is very promising for its high energy density, for various applications such as automotive and power tools.
However, there are several critical issues for lithium-rich NMC. Firstly, the requirements for compatible electrolyte systems for lithium-rich NMC are strict. As described above, when lithium-rich NMC is used as cathode material in a full cell it needs to be activated by charging to above 4.5 V (corresponding to 4.6 V vs. Li/Li+) to obtain the high capacity above 300 mAh/g. Then it also needs a broad cycling range, usually 2˜4.6 V (vs. Li/Li+) to keep the reversible capacity around 250 mAh/g. Usually such a high voltage cannot be tolerated by the current electrolyte systems: the organic solvents in the electrolyte, which are mainly linear and cyclic carbonates, start to decompose at high voltages >4.5 V, and form side products, which negatively impact the cathode/electrolyte and anode/electrolyte interface. The side products deteriorate the electrochemical performance of the batteries and result in a strong fading of the capacity. Research on improving the stability of the electrolytes at high voltage>4.5 V is ongoing, including finding new solvents, inventing new salts and combining functional additives.
There are also other intrinsic drawbacks for lithium-rich NMC. There are wide studies that show that during cycling, the average discharge voltage of lithium-rich NMC is gradually decreasing. In J. Electrochem. Soc. 2014 161(3): A318-A325, Croy et al. studied the voltage fading mechanism of lithium-rich NMC. They also found that the voltage fading results from the migration of transition metals to the lithium layers, thereby changing the local structure and causing the decrease of the energy output.
There are many efforts to improve the voltage fading of lithium-rich NMC. In J. Electrochem. Soc. 2015 162(3): A322-A329, Lee et al. found there is no benefit for improving the voltage fading by Al and Ga doping. In another work done by Bloom at al. in J. Power. Sources 2014 249: 509-514, they confirmed that coating approaches, for example, Al2O3, TiO2 and AlPO4 coating on lithium-rich NMC, does not help to suppress the voltage fading.
In EP2654109A1, WO2015/040818, JP5459565B2 and EP2557068A1 overlithiated zirconium-doped nickel-manganese-cobalt oxides and their use as positive electrode materials for secondary batteries is disclosed. In EP2826750A1 and JP2014-049309 positive electrode materials comprising a zirconium-doped lithium nickel-cobalt-manganese oxide and a lithium zirconate compound that is formed on its surface are disclosed.
In view of the problems cited before, the search for effective approaches to improve the voltage stability of lithium-rich NMC during cycling becomes one of the most important topics for the future development and application of this type of cathode material. An object of the present invention is to provide solutions to suppress the voltage fading of lithium-rich NMC while keeping the high gravimetric energy density of the materials.